Linear regulators are commonly used for step-down (e.g. the output supply voltage is lower than input source voltage) applications.
An advantage of integrated circuit ‘IC’ linear regulators is simple implementation and minimal parts and low output signal ripple, although output signal ripple is still present. A disadvantage of linear regulators is low efficiency. Significant power is dissipated within the linear regulator IC, as the converter is constantly on and conducting current. Linear regulators are normally used when the difference between input source voltage and output supply voltage is minimal, and converter efficiency is not a concern.
Switching voltage regulators are commonly used for both step-up and step-down applications, and differ from linear regulators by means of pulse-width modulation (PWM) implementation. Switching regulators control the output voltage by using a current switch with a constant frequency and variable duty-cycle. Switching frequencies are generally from a few kHz to a few MHz. The switch duty-cycle ratio determines how much and how quickly the output supply voltage increases or decreases, depending on the load state and input source voltage. Some switching regulators utilize both variable switching frequency and duty-cycle.
An advantage of switching regulators is efficiency, as minimal power is dissipated in the power path (often FET switches) when the output supply voltage is sufficient for the load state. Essentially, the power converter “shuts off” when power is not needed, due to minimal switch duty-cycle. A disadvantage of switching regulators is complexity, as several external passive components are required.
Regulators of both types suffer from output signal ripple, which may be handled with bypass capacitance and/or inductance near at least the load. However, the use of passive components such as capacitors and/or inductors at the output does not fully resolve the problem of output signal ripple due to the capacitance or inductance characteristics of the passive components themselves and/or other circuit effects.
A voltage regulator is designed to maintain a constant voltage level within a specified tolerance range, even when the load current varies. FIG. 1 shows a structure of a typical switching voltage regulator 10. It comprises a power supply 12, a power conversion stage (PCS) 14 and a controller stage 16. The controller stage 16 senses the output voltage vo of the voltage regulator 10, compares it with a reference vref of the voltage regulator, receives some system variables such as inductor current, switch voltage, switch current, etc, and uses these to derive a control signal to command a control variable, typically duty cycle, to the PCS 14, so as to regulate vo to converge it to vref. In this example of a typical voltage regulator, there is provided an output passive component comprising a capacitor ‘C’ 18 connected in parallel with a load impedance ‘ZL’ 20 of the voltage regulator 10. A linear regulator has a similar construction and mode of operation although it will be understood that the PCS of a linear regulator does not have an inductor, a switch voltage and a switch current, but instead has a power transistor operating in linear mode only.
Since the 1970's, many research articles have been devoted to different control schemes such as voltage-mode control, current-mode control, nonlinear control, one-cycle control, sliding-mode control, etc. for the controller. To further improve voltage regulation capability and dynamic response, low dropout regulators (LDRs) have been widely used in post regulation of switching regulators. LDRs are linear regulators operating with very small input-output differential voltage. Although they have low power loss, they necessitate processing the entire load power of the voltage regulator. The series pass element in the LDRs has to carry the whole of the voltage regulator load current. Consequently, LDRs as a means of compensating the output signal of a voltage regulator are more suitable for low-power voltage regulation applications.